The invention relates to an integrated broadly tunable optical filter designed, in particular, for advanced WDM (Wavelength Division Multiplexing) applications as well as for use in spectroscopic testing of various optical components.
Broadly tunable optical filters are key components in optical communication systems. Two objectives dominate with such filters. The aims, on the one hand, are a broad tunable range and, on the other hand, high spectral selectivity. The planar integrated structures known hitherto, however, permit only one of these objectives at a time to be met.
A conventional filter structure, known as co-directional coupler structure, is described in "Broadly tunable InGaAsP/InP buried rib waveguide vertical coupler filter" (R. C. Alferness, L. L. Buhl, U. Koren, B. I. Miller, M. G. Young, T. L. Koch, G. A. Burrus, G. Raybon), Appl. Phys. Lett. 60(8), 24 Feb. 1992. This conventional structure consists of two asymmetrical waveguides having different effective refractive indices. The optical signal launched into the uppermost waveguide is launched into the lowermost waveguide and selectively reflected towards the uppermost waveguide. If the parameters of the waveguides are chosen carefully, a structure of this type can act as a selective filter.
The advantage of this conventional structure is the fact that a broad tuning range is provided, but it is difficult to obtain high spectral selectivity without constructing a long appliance.
On the other hand, a semiconductor optical structure is also known which can provide high selectivity. This conventional structure makes use of a waveguide having a grating grown thereon, that grating of which is periodically omitted. A structure of this type has already been used in a tunable laser (V. Jayaraman, D. A. Cohen, L. A. Coldten, "Demonstration of broadband tunability in a semiconductor laser using sampled gratings", Appl. Phys. Lett 60(19), 11 May 1992). Nevertheless, a structure of this type has hitherto not been used in a filter structure.
This conventional structure provides a comb-shaped reflection spectrum. The parameters thereof are the spacing between the peaks, the spectral bandwidth of the envelope of the comb spectrum, the peak maximums and the bandwidth of a reflection peak. The latter is probably the most important parameter, because it affects the selectivity of the structure. Most unfortunately, this parameter too limits the tuning range of the structure.